This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-007815, filed Jan. 17, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to an X-ray computer tomography apparatus, and more particularly to a thin-type X-ray computer tomography apparatus (hereinafter, referred to as an X-ray CT apparatus) provided with a small-sized high-voltage transformer capable of supplying power and stepping up the voltage by itself in a noncontacting manner.
In the field of X-ray CT apparatuses, high-speed tomographic techniques for turning the rotatable gantry section with respect to the static gantry section at high speed are being developed rapidly. With the high-speed tomographic techniques, not only a larger amount of image information can be obtained in a short time but also the time during which the subject is tied down. Thus, the techniques are very effective in a group medical examination as well as an ordinary physical examination. In recent years, a high-speed rotation of about 0.5 second has already been put to practical use and further an ultrahigh-speed rotation of less than 0.3 second is gradually gaining practicality.
Because of the problem of centrifugal force acting on the rotatable gantry section by high-speed rotation, the gantry of the X-ray CT apparatus have been required to be made smaller in size, particularly thinner along the axis of the body of the subject. Although various measures have been taken to improve the method of arranging the units in the rotatable gantry section, it is necessary to make each unit as small as possible to reduce the size of the rotatable gantry section itself. In the X-ray CT apparatus, the top on which the subject has been laid is inserted into a cylindrical space formed within the rotatable gantry section and then pictures are taken. In such an X-ray CT apparatus, it is necessary to improve the accessibility of the subject as in the magnetic resonance imaging apparatus. An improvement in the accessibility enables various medical procedures when the subject is inserted into the apparatus. Moreover, the subject has a less feeling of confinement in the cylindrical space.
The power supply from the static gantry section to the rotatable gantry section is carried out in a contacting manner or a noncontacting manner. One example of supplying power in a contacting manner is achieved by using a slip ring mechanism. As is well known, the slip ring mechanism has a brush provided on the rotatable gantry section and causes the brush to come into contact with the slip ring provided on the static gantry section, thereby supplying power from the static gantry section to the rotatable gantry section. The slip ring mechanism is considered to be unsuitable for high-speed rotation, because the friction between the brush and the slip ring produces heat and abrasion powder. In addition, since there is a possibility of electric discharges, the slip ring is regarded as unsuitable for power transmission of such a high voltage, for example, 10 kV or more as is applied across both ends of an X-ray tube. Under these conditions, several concepts of X-ray CT apparatuses that supply power from the static gantry section to the rotatable gantry section in a noncontacting manner have been proposed.
One known noncontacting-type X-ray CT apparatus is disclosed in U.S. Pat. No. 4,912,735. This X-ray CT apparatus supplies power in a noncontacting manner by electromagnetic induction.
FIG. 11 is a schematic circuit diagram of a conventional X-ray CT apparatus that supplies power from the static gantry section to the rotatable gantry section. FIG. 12 shows the location of the individual component parts. In FIG. 12, an AC/DC converter 14b is connected to an alternating-current (a.c.) power source 11 provided on the side face of the lower part of the inside of the static gantry section 111. The output terminal of the AC/DC converter 14b is connected to an inverter 15. The output of the inverter 15 is connected to the primary coil 116 of the static gantry section 111. The primary coil 116 is wound around the cylindrical static gantry section 111 in such a manner that it surrounds the outer surface of the static gantry section 111. The rotatable gantry section 112 has a cylindrical shape as the static gantry section 111 does and is provided on the static gantry section 111 on the same central axis of the cylinder in such a manner that it can rotate. On the rotatable gantry section 112, a secondary coil 119 is provided in a position facing the primary coil 116 of the static gantry section 111. Like the primary coil 116, the secondary coil 119 is wound around the rotatable gantry section 112 in such a manner that it surrounds the outer surface of the rotatable gantry section 112. A high-voltage transformer 113 is connected to the secondary coil 119. A rectifier 20 is connected to the output terminal of the high-voltage transformer 113. An X-ray tube 21 is connected to the output terminal of the rectifier 20. A magnetic field generated at the primary coil 116 induces power at the secondary 119. The electromagnetic induction enables power to be supplied from the static gantry section 111 to the rotatable gantry section 112.
The conventional X-ray CT apparatus with the above configuration that supplies power in a noncontacting manner has the following problem.
As compared with an ordinary transformer where the cores are integrally formed, the leakage inductance between the cores of the separate primary coil 116 and secondary coil 119 is greater, impeding a high-frequency operation, which makes it difficult to miniaturize the unit. The miniaturization is possible only when the operation of the unit is carried out at higher speed. For this reason, in general, to reduce the leakage inductance, the primary coil 116 is arranged as close to the secondary coil 119 as possible or the coil windings are wound even in the grooves of the cores, thereby improving the degree of coupling. In such a manner of improving the degree of coupling, there arises a problem that realizing the high frequency operation by overcoming the leakage inductance between the separated cores is limited. Moreover, it makes high-voltage insulation difficult from the viewpoint of manufacturing techniques. Thus, to obtain a high voltage of about 75 kV to 150 kV on the secondary side, it is necessary to provide an additional high-voltage transformer 113. This puts significant restriction on the rotatable gantry section being made smaller and thinner. In the X-ray CT apparatus, the primary coil 116 is wound around the cylindrical static gantry section 111 in such a manner that it surrounds the outer surface of the static gantry section 111 and the secondary coil 119 is wound around the rotatable gantry section 112 in such a manner that it surrounds the outer surface of the rotatable gantry section 112, with the result that the distance between the windings facing each other is relatively long. This makes the parasitic capacitance large, making a high-frequency operation difficult, which is one of the causes of the difficulty in making the unit smaller and thinner.
Another known noncontacting X-ray CT apparatus is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-204192 and Jpn. Pat. Appln. KOKAI Publication No. 8-336521. Each of these conventional X-ray CT apparatuses has the following configuration.
Each of the X-ray CT apparatuses comprises electromagnetic induction transmission means including a first winding provided on the fixed frame of a scanner and a second winding provided on the rotary section of the scanner in such a manner that it faces the first winding, and a high-voltage generator connected to the electromagnetic induction transmission means. Each of the X-ray CT apparatuses supplies specific power in a noncontacting manner by electromagnetic induction.
Each of these conventional X-ray CT apparatuses is provided with an additional high-voltage transformer to obtain a high voltage, because of the problem of the leakage inductance, as explained in the X-ray CT apparatus disclosed in U.S. Pat. No. 4,912,735. This is a serious hindrance in making the rotatable gantry section smaller and thinner.
Each of U.S. Pat. No. 5,105,351 and U.S. Pat. No. 5,272,612, which are assigned to the same assignee as the present invention, discloses a device for applying a high voltage to an X-ray tube. Each of these two devices includes a plurality of high-voltage transformers, taking the size reduction of transformers into account. However, there is no reference to noncontacting power supply by electromagnetic induction or a concrete application of the inventions to an X-ray CT apparatus.
The object of the present invention is to provide a thin-type and small-sized X-ray computer tomography apparatus.
According to the present invention, there is provided an X-ray computer tomography apparatus comprising: a static gantry section; a rotatable gantry section which is provided on the static gantry section in a rotatable manner and has an X-ray tube for generating X rays; a frequency converting circuit which is connected to an alternating-current power source and converts the output voltage from the alternating-current power source into a desired high-frequency voltage; a high-voltage transformer which transmits the output of the frequency converting circuit from the static gantry section to the rotatable gantry section and steps up the output to a desired high voltage; and a rectifier circuit which converts the alternating-current voltage outputted from the high-voltage transformer into a direct-current voltage and supplies the direct-current voltage to the X-ray tube, wherein the high-voltage transformer including: a primary-side which is provided on the static gantry section and to which the output of the frequency converting circuit is supplied, and a resonance circuit with a capacitor connected to a winding of a secondary-side, which is provided on the rotatable gantry section and generates the high voltage.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.